DAPIJNIA : A STUDY OF THE FOOD OF FIHHEH 155 



or in April they begin to increase, reaching a maximmn in May. 

 During the midsummer a marked decline occurs, to be followed 

 by a rise at the end of September or in October ; after that 

 the numbers decline for the winter. The reason for these two 

 maxima is that the spring and autumn are periods of rapid 

 growth of the food of the Entomostraca, and this in turn is due 

 to the mixing up of the waters at those periods, and conse- 

 quently of the salts and organic stuff upon which, in last 

 analysis, the Entomostraca depend. This mixing up of the 

 waters depends on the variation of the density of water 

 as its temperature changes. Water is heaviest at 4° C. 

 above the freezing-point and tends to settle below water 

 that is warmer or colder than 4° C In the spring the bot- 

 tom waters remain near the freezing-point (0° C), while the 

 surface temperature rises. As it approaches 4° the surface 

 water grows heavier and falls to the bottom, forcing the colder 

 water up, and this goes on until the whole mass of the water 

 has reached the temperature of 4°, — this constitutes the spring 

 circulation of the water ; after that, the surface water, becom- 

 ing still warmer, remains on top. In the autumn the whole 

 mass of water, even to the bottom, has at first a higher tem- 

 perature than 4° C, but as the frost comes the surface water 

 gredually approaches 4° C. and begins to fall, forcing the 

 warmer, lighter water to the surface. Eventually the densest 

 (4° C.) water is at the bottom, while the surface water freezes 

 (at 0° C.) and floats on top. This is the autumnal movement 

 of the waters. By means, then, of these remarkable vertical 

 movements of the waters of lakes the bottom food is made 

 available for the surface swarms of Entomostraca. 



Entomostraca live not only in fresh water, but also in the 

 sea — that greatest consumer of the sun's energy. Living 



